Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

And from what I have seen, the chances of being in an accident on a bike are quite low, and the chances of being in an accident and getting a severe head injury are lower still.

Not sure I agree with this.

I have tumbles quite regulary (well in the winter at least). Most of these are low speed tumbles due to poor conditions, and any head impact (rare) is nicely cushioned by my helmet. The formentioned helmet now has a notable dent in it from a high speed tumble a couple of weeks ago (now buying new one).

Using your 'risk matrix' I'd say....
slight tumble chances x slight tumble potiental injury = quite high.

Thats just me though. I still believe in personal choice.